1) Field of the Invention:
The present invention relates to a production method of foamed particles of an uncrosslinked ethylene-based resin, and more particularly to a production method of foamed particles of an uncrosslinked linear low-density polyethylene resin.
2) Description of the Related Art:
As a foaming process of resin particles, it has heretofore been known to disperse particles of a synthetic resin in a dispersion medium such as water in the presence of a volatile foaming agent in a closed vessel, to heat the resultant dispersion to a temperature of at least the softening temperature of the resin particles while keeping the internal pressure of the vessel at least the vapor pressure of the foaming agent, and then to open the vessel at one end thereof so as to release the resin particles and the dispersion medium into an atmosphere of a pressure lower than the internal pressure of the vessel.
As exemplary volatile foaming agents useful in the practice of this process, there have heretofore been widely used halogenated hydrocarbons such as trichlorofluoromethane and dichlorodifluoromethane. However, most of these compounds conventionally used as the foaming agents involve in the circumstances a problem of ozonosphere destruction, or of impracticability due to their expensiveness though little problem of ozonosphere destruction is offered, and/or the like.
Besides, the volatile foaming agents swell the synthetic resin particles, so that the appropriate range of foaming temperature upon their foaming is limited. Therefore, they also involve a problem that the foaming temperature greatly affects the expansion ratio of the resin particles, so that difficulties are encountered on controlling of the expansion ratio.
Many studies were made to solve such problems. The present applicants also carried out an extensive investigation with a view toward solving such problems. As a result, it was previously proposed to use, as a foaming agent, an inorganic gas such as carbon dioxide, which hitherto had no regard for the foaming agent, so as to obtain foamed particles of a synthetic resin (for example, Japanese Patent Publication No. 61227/1987, Japanese Patent Application Laid-Open Nos. 2741/1986 and 4738/1986, etc.).
However, the use of the inorganic gas as a foaming agent involved a problem that foamed particles high in expansion ratio are difficult to stably provide because the impregnating ability of the foaming agent into the synthetic resin particles is poor, and the effect to plasticize the resin and the impregnation speed of the gas vary compared with the volatile foaming agents, and so on. Therefore, the production of foamed particles on the industrial scale has still involved many problems that have to be solved.
The present applicants carried out an extensive investigation with a view toward solving the above problems. As a result, it was found that When an inorganic material such as borax, aluminum hydroxide or zeolite is contained in resin particles, foamed particles high in expansion ratio can be obtained even when an inorganic gas is used as a foaming agent to produce the foamed particles on an industrial scale, and moreover the amount of a volatile foaming agent can be lessened and foamed particles high in expansion ratio can be obtained with a smaller amount of the foaming agent more stably than the method described in Japanese Patent Application Laid-Open No. 4738/1986 even when the conventional volatile foaming agent is used. Applications for patent were thus filed previously (Japanese Patent Application Laid-Open Nos. 166238/1991 and 223347/1991, etc.).
However, when the resin particles containing the above-described inorganic material were impregnated with a foaming agent of the inorganic gas type, in particular, a foaming agent comprising, as a principal component, carbon dioxide to conduct foaming, there were generally problems that cells in the resulting foamed particles tend to become fine, and scatter tends to occur in cell diameter though a difference more or less arises depending on the kind of the inorganic material to be contained. For example, when aluminum hydroxide, zeolite, silica or the like was contained as an inorganic material in resin particles, no scatter occurred in cell diameter, but cells tended to become fine. On the other hand, when borax or the like was contained as an inorganic material, high expansion ratio could be achieved, and a tendency for cells to become fine became less than the case where the above-described inorganic material was contained as the inorganic material, but there was a problem that scatter of cell diameter occurs. Such foamed particles that the scatter of cell diameter was wide, or the cells became fine involved problems that secondary expandability upon their molding is inferior, and the dimensional accuracy of the resulting molded article is poor.
The present applicants further carried out an extensive investigation with a view toward solving the above-described problems. As a result, it was found that when the particle size of the inorganic material to be contained in the resin particles is specifically limited, the scattering of cell diameter can be prevented, so that foamed particles having even cells can be obtained (Japanese Patent Application Laid-Open No. 359037/1992).
In a method in which an inorganic material such as talc, zeolite or borax is added into resin particles to increase the expansion ratio of the resulting foamed particles, the inorganic material to be contained in the resin particles is preferably borax for obtaining foamed particles having a higher expansion ratio. However, if a base resin for resin particles was linear low-density polyethylene, effects to sufficiently improve both increase of expansion ratio and scatter of cell diameter could not be expected by the addition of borax like resin particles comprising, as a base resin, crosslinked low-density polyethylene or a propylene-based resin even when the particle size of borax was controlled to a certain range according to the method in Japanese Patent Application Laid-Open No. 359037/1992.